[racket] Untyped arithmetics performance

Hi, I'm observing the following difference in performance:

> (time (exact->inexact (for/sum ([x (in-range 10000)]) (/ x 10000))))
cpu time: 6 real time: 6 gc time: 0
4999.5
> (time (exact->inexact (for/sum ([x (in-range 10000)]) (/ x (- 10000 x)))))
cpu time: 3624 real time: 3622 gc time: 39
87876.06036044382

The difference seems huge to me and I'm wondering whether it's "normal" and what are the right tools to overcome it.
If I understand it correctly, it's about contract checking and the fact that there is one more math opeartion involved does not in itself lead to such difference. The division operator needs to be sure that the outcome of the minus op. is of the right type, is that correct?

If so -- and the increase in time cannot be considered a "bug" -- what is the right way to get around that. I've employed 2 ways: Typed Racket and switching the portion of code to racket/flonum. Both of these approaches resulted in big perf. improvement and basically make the computation instant again. Are these the right, "rackety", solutions? I've also tried to use unchecked ops but Racket 6.1.1 is constantly segfaulting on me when running this (from my digging I think unsafe-fl/ is what segfaults it):

(time (exact->inexact (for/sum ([x (in-range 10000)]) (unsafe-fl/ (->fl x) (unsafe-fx- 10000 x)))))

Daniel